# std::optional<T>::operator=

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optional& operator=( std::nullopt_t ) noexcept; | (1) | (since C++17) (constexpr since C++20)
constexpr optional& operator=( const optional& other ); | (2) | (since C++17)
constexpr optional& operator=( optional&& other ) noexcept(/* see below */); | (3) | (since C++17)
template< class U = T > optional& operator=( U&& value ); | (4) | (since C++17) (constexpr since C++20)
template< class U > optional& operator=( const optional<U>& other ); | (5) | (since C++17) (constexpr since C++20)
template< class U > optional& operator=( optional<U>&& other ); | (6) | (since C++17) (constexpr since C++20)

Replaces contents of *this with the contents of other.

1) If *this contains a value before the call, the contained value is destroyed by calling its destructor as if by value().T::~T(). *this does not contain a value after this call.
2,3) Assigns the state of other.

- If both *this and other do not contain a value, the function has no effect.
- If *this contains a value, but other does not, then the contained value is destroyed by calling its destructor. *this does not contain a value after the call.
- If other contains a value, then depending on whether *this contains a value, the contained value is either direct-initialized or assigned from *other (2) or std::move(*other) (3). Note that a moved-from optional still contains a value.
- Overload (2) is deleted when either std::is_copy_constructible_v<T> or std::is_copy_assignable_v<T> is false. It is trivial if std::is_trivially_copy_constructible_v<T>, std::is_trivially_copy_assignable_v<T> and std::is_trivially_destructible_v<T> are all true.
- Overload (3) does not participate in overload resolution when either std::is_move_constructible_v<T> or std::is_move_assignable_v<T> is false. It is trivial if std::is_trivially_move_constructible_v<T>, std::is_trivially_move_assignable_v<T> and std::is_trivially_destructible_v<T> are all true.

4) Perfect-forwarded assignment: depending on whether *this contains a value before the call, the contained value is either direct-initialized from std::forward<U>(value) or assigned from std::forward<U>(value). The function does not participate in overload resolution unless std::decay_t<U>(until C++20)std::remove_cvref_t<U>(since C++20) is not std::optional<T>, std::is_constructible_v<T, U> is true, std::is_assignable_v<T&, U> is true, and at least one of the following is true:

- `T` is not a scalar type;
- std::decay_t<U> is not `T`.

5,6) Assigns the state of other.

- If both *this and other do not contain a value, the function has no effect.
- If *this contains a value, but other does not, then the contained value is destroyed by calling its destructor. *this does not contain a value after the call.
- If other contains a value, then depending on whether *this contains a value, the contained value is either direct-initialized or assigned from *other (5) or std::move(*other) (6). Note that a moved-from optional still contains a value.
- These overloads do not participate in overload resolution unless the following conditions are met:
  - `T` is not constructible, convertible, or assignable from any expression of type (possibly const) std::optional<U>, i.e., the following 12 type traits are all false:
    - std::is_constructible_v<T, std::optional<U>&>
    - std::is_constructible_v<T, const std::optional<U>&>
    - std::is_constructible_v<T, std::optional<U>&&>
    - std::is_constructible_v<T, const std::optional<U>&&>
    - std::is_convertible_v<std::optional<U>&, T>
    - std::is_convertible_v<const std::optional<U>&, T>
    - std::is_convertible_v<std::optional<U>&&, T>
    - std::is_convertible_v<const std::optional<U>&&, T>
    - std::is_assignable_v<T&, std::optional<U>&>
    - std::is_assignable_v<T&, const std::optional<U>&>
    - std::is_assignable_v<T&, std::optional<U>&&>
    - std::is_assignable_v<T&, const std::optional<U>&&>
  - For overload (5), std::is_constructible_v<T, const U&> and std::is_assignable_v<T&, const U&> are both true.
  - For overload (6), std::is_constructible_v<T, U> and std::is_assignable_v<T&, U> are both true.

### Parameters

other | - | another `optional` object whose contained value to assign
value | - | value to assign to the contained value

### Return value

*this

### Exceptions

2-6) Throws any exception thrown by the constructor or assignment operator of `T`. If an exception is thrown, the initialization state of *this (and of other in case of (2,3) and (5,6)) is unchanged, i.e. if the object contained a value, it still contains a value, and the other way round. The contents of value and the contained values of *this and other depend on the exception safety guarantees of the operation from which the exception originates (copy-constructor, move-assignment, etc.).
3) Has following
`noexcept` specification:
noexcept(std::is_nothrow_move_assignable_v<T> && std::is_nothrow_move_constructible_v<T>)

### Notes

An optional object `op` may be turned into an empty optional with both op = {}; and op = nullopt;. The first expression constructs an empty optional object with {} and assigns it to `op`.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_optional` | `202106L` | (C++20) (DR20) | Fully constexpr (1), (4-6)

### Example

Run this code

    #include <iostream>
    #include <optional>

    int main()
    {
        std::optional<const char*> s1 = "abc", s2; // constructor
        s2 = s1; // assignment
        s1 = "def"; // decaying assignment (U = char[4], T = const char*)
        std::cout << *s2 << ' ' << *s1 << '\n';
    }

Output:

    abc def

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
P0602R4 | C++17 | copy/move assignment operator may not be trivial even if underlying operations are trivial | required to propagate triviality
P2231R1 | C++20 | converting assignment operators (1) and (4-6) were not constexpr while the required operations can be in C++20 | made constexpr

### See also

emplace | constructs the contained value in-place (public member function)

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